my brush setup got destroyed when open in node graph
The Texture Nodes Editor is pretty useful if you are working with making brushes except that it isn't safe when setting up a brush before enabling nodes. Pretty much all the time I use it, I set up the brush in the node editor before adding anything in the texture stack. The default when starting is the red and white checkered node, and from there you can add what you want and build a pretty complicated setup - but if you try to nodify your existing brush, it will negate all the previous effort, so start in the node editor if that is what you want.

You don't normally need to though unless you are combining procedurals together as a single brush/brush mask.

Edit: For 2.80 beta, we have seen this node system removed for now. There are possibilities after Everything Nodes is worked on that we will see and even more robust system for manipulating texture brushes with nodes and using them in the texture paint system.